This invention relates generally to optical materials and more particularly to materials which have improved optical transmittance for far infrared electromagnetic radiation.
As is known in the art, optically transmittant materials are used in fabricating optical windows for use in various optical detection applications. In particular, such materials are used as domes and windows in heat seeking missile applications. When used in such applications, the selected material for a given thickness must be sufficiently transmittant to infrared electromagnetic radiation to enable sensitive detection of such radiation, and must be sufficiently durable to withstand mechanical forces encountered during the missile's flight. Further, the selected material must also be resistant to rain erosion, damage caused by high velocity water droplets impinging on the material during flight.
One material which has been suggested for such application is calcium lanthanum sulfide (CaLa.sub.2 S.sub.4). Calcium lanthanum sulfide is one of a class of materials known as ternary sulfides. These materials are characterized by having two cations in chemical combination with a sulfide S.sup.2- ion as the anion of the compound. One method used in the prior art for fabricating such ternary sulfide material as described in an article, by Pennsylvania State University, entitled "Ternary Sulfide Infrared Window Materials," First Annual Report to the Office of Naval Research, Contract Number N00014-80-C-0526 (May 1, 1980-October 31, 1981), AD-A120 153/2 Index 4 February 1983, includes the step of spray drying calcium and lanthanum ions from a solution of calcium nitrate and lanthanum nitrate in an atmosphere of oxygen to form a starting powder of calcium oxide powder and lanthanum oxide powder. The starting powder is then sulfurized to provide calcium lanthanum sulfide powder. The resulting powder is further processed to form a compact, densified body of calcium lanthanum sulfide material. While the spray drying of calcium and lanthanum ions is an adequate process for producing calcium lanthanum sulfide material, such a process may be costly, and further, the degree of optical transmittance provided by such material is less than the theoretical maximum transmittance. A second method known in the art to produce a calcium lanthanum sulfide compound is a technique commonly referred to as solid state reaction processing. This involves producing a starting powder by mixing powders of calcium carbonate and lanthanum oxide, and sulfurizing the starting powder to produce the calcium lanthanum sulfide material. While this latter method is less costly and simpler compared with the former method; material produced by the latter method has reduced transmittance. More particularly, with the latter method an x-ray diffraction pattern of the produced calcium lanthanum sulfide material indicates the presence of the binary sulfide, calcium sulfide. Since calcium sulfide has a substantially different crystallographic structure than calcium lanthanum sulfide, the presence of calcium sulfide is believed to cause the reduced transmittance of calcium lanthanum sulfide material produced with the latter method. Further, solid state reaction processing of commercially available calcium carbonate and lanthanum oxide powders provides a starting powder having a very large variation in powder particle size. Due to the presence of very large agglomerated particles of lanthanum oxide, the sulfurization process is very slow, necessitating the use of a particle size reduction process such as jet milling prior to sulfurization. Jet milling, or similar techniques of particle size reduction, reduces some of the cost advantages of the latter method over the former method and also introduces impurities into the powder which are believed to degrade optical properties of the resulting calcium lanthanum sulfide.
With calcium lanthanum sulfide powder formed by either of the above methods, the powder is then molded into a compact body and then the body is densified to provide the compact, densified body. Several different densifying steps are generally performed in order to densify the material. In general, however, powders fabricated by either method described above generally require the step of conventional hot pressing in the densification process to provide bodies with high density. Hot pressing involves compression of the compact body in a single or uniaxial direction at an elevated temperature. While hot pressing is a conventional processing technique, its elimination is highly desirable because, in general, hot pressing can be performed only on a single body at a time thus increasing product cost.